How do I create a property in a panel to edit particle systems?
It would be something like:

particles = obj.modifiers.get("ParticleSystem 1")
if particles:
    row = layout.row()
    row.prop(particles.particle_system.settings, "count")


or

particles = obj.particle_systems.get('ParticleSystem')
if particles:
    row = layout.row()
    row.prop(particles.settings, "count")